
With the availability of Serial Attached SCSI (SAS) components, the next generation of business class storage technology has arrived. SAS, however, is not only the legitimate successor to UltraSCSI, but it will also enable new applications while boosting scalability to unprecedented levels. To demonstrate SAS' potential, we took a look at the technology, host adapters, hard drives and storage appliances.
SAS is not something entirely new, but it merges the best of two worlds. The first part is the serialized transmission of data, which requires far fewer physical connections. Moving from parallel to serialized operations has eliminated the use of a bus as well.Although the current SAS specification defines a 300 MB/s bandwidth per port, which is less than UltraSCSI's 320 MB/s, exchanging the sharedbus by point-to-point connections between devices is a huge advantage. The second important ingredient for SAS is the SCSI protocol, which is popular and powerful.
In addition to the interface specifics, SAS can take advantage of a large variety of RAID implementations. Storage giants such as Adaptec or LSI Logic offer comprehensive features that allow for expansion, transformation, migration, nesting and concurrency of RAID arrays across multiple controllers and drives.
Though almost being metamorphoses, most of these actions can even be performed on the fly today. In this context we should also mention the names AMCC/3Ware, Areca and Broadcom/Raidcore, because they helped move enterprise class features into the SATA space.
Compared to SATA, conventional SCSI is losing ground on all fronts, except for high-end enterprise applications. SATA offers adequate hard drives, attractive pricing and a wide selection of solutions. Another really smart feature of SAS is that it can be integrated with existing SATA infrastructures, which can be accommodated by SAS host adapters. However, SAS drives cannot run on SATA adapters.

Source: Adaptec.
First of all, we believe it is important to understand SAS' history. SCSI, the small computer system interface, has always served as a professional bus for attaching storage and certain other device types to computers. Workstation and server class hard drives were and still are based on SCSI technology. In contrast to mainstream ATA with only two drives per controller port, SCSI allows for daisy-chaining up to 15 devices and it comes with a powerful command protocol. Devices require a unique SCSI ID, which is either set manually on each device, or it can be assigned via SCAM (SCSI Configuration Automatically). Since IDs are not necessarily unique across two or more SCSI adapters, so-called LUNs (Logical Unit Numbers) are put together to identify units across complex SCSI environments.
SCSI hardware is more flexible and robust than ATA (also called IDE, which stands for Integrated Drive Electronics). Devices can be attached internally or externally and use cables at lengths of up to 39.37 ft (12 m) as long as the bus is terminated properly, which eliminates signal reflection. As SCSI evolved, multiple standards became available, covering different bus widths, clock speeds, connectors and voltage signaling (Fast, Wide, Ultra, Ultra Wide, Ultra2, Ultra2 Wide, Ultra3, Ultra320 SCSI). The good part is they all share a common command set.
All SCSI communication takes place between an initiator (host adapter) that sends commands, and a target drive thatresponds. Immediately after receiving a command sequence, the target returns a so-called sense code (busy, error or success check condition), so the initiator knows whether or not it will get the response it wants.
SCSI has almost 60 different commands. These are classified into four categories: non-data, bi-directional, read and write data commands.
The limitations of SCSI become obvious as you keep adding drives to the bus. While few hard drives do not saturate the bandwidth of Ultra320 SCSI (320 MB/s), five or more drives easily do. Adding a second host adapter to balance the load is a solution, but it adds cost. In addition, cramming multiple drives into a server system is neither elegant nor advisable. Cables are also an issue; the twisted 80-pin models are very expensive. If you want drives to be hot-swappable, e.g. for easy replacement of defective drives, backplanes are required for proper removal and insertion of devices into the SCSI chain.
Ideally, your drives should be placed into separate storage enclosures or appliances that usually include all the hot-swapping capabilities as well as managing features. Altogether there are lots of professional SCSI-based solutions available. But all of these come at a price premium, which is why SATA has had tremendous impetus over the last couple of years. Though SATA will never be adequate for high-end enterprise requirements, it perfectly complements SAS when it comes to creating new, scalable solutions for next-generation networked environments.

SAS does not share one bus for multiple devices. Source: Adaptec.

On the left side is the SATA data connector. The right connector is for the power supply. The number of pins is required to provide 3.3 V, 5 V and 12 V to each SATA device.
SATA is in its second generation and has been available for many years. SATA I defines a gross bandwidth of 1.5 Gb/s by using two serial links running low-voltage differential signaling, while 8/10-bit encoding at the physical layer (10 bits are transferred for 8 data bits) results in a maximum interface throughput of 150 MB/s. During the rollout of SATA at 300 MB/s, people started calling this acceleration SATA II, although the standardizing SATA-IO (International Organization) intended to add more features prior to calling it SATA II. The latest specification hence is SATA 2.5, and it includes SATA extensions such as Native Command Queuing (NCQ) and eSATA (external SATA), port multipliers (up to four drives per port) etc. However, SATA features are optional both on the controller and the hard drive side.
Hopefully, SATA III at 600 MB/s will stop growing by 2007.
While parallel ATA (UltraATA) cables were limited to 18" (46 cm), SATA cables may be up to 39" (1 m), with double the cable length for eSATA. Rather than 40 or 80 wires, only seven pins are required for serial data transmission. Obviously, SATA cables are pretty narrow, easy to run inside of systems and they will not restrict airflow either. There is only one device per SATA port, making the interface a point-to-point connection.

SATA connects data and power via separate plugs.



The signaling equals to what is used in SATA. Source: Adaptec.
Here comes the neat thing: Serial Attached SCSI supports both SCSI and SATA, which enables SAS controllers to run either SAS or SATA devices (or both). However, SAS devices cannot be operated on SATA controllers because the Serial SCSI Protocol (SSP) is used. Like SATA, SAS follows the principle of point-to-point links to the drives (currently 300 MB/s), while there is the option to use SAS expanders for attaching more drives than SAS ports available. SAS hard drives are dual-ported, each featuring his unique SAS ID, which means that it is possible to use two physical data connections to provide redundant data paths to two different hops/hosts. Thanks to STP (SATA Tunneling Protocol), SAS controllers will still be able to talk to SATA drives that are attached to an expander.

Source: Adaptec

Source: Adaptec

Source: Adaptec
A single SAS link to an expander that hosts multiple drives would, of course, be an obvious bottleneck, so the specification provides for wide SAS ports. A wide port groups several SAS links into one interlink between any two SAS devices (usually a host controller and an expander). Links to attached expanders can be scaled up as your storage requirements grow. However, redundant connects are not allowed as this is taken care of by wide links; any other sort of loops or rings are not supported, either.

Source: Adaptec
Future SAS implementations will provide 600 and 1,200 MB/s bandwidth per port. As hard drive performance is not going to increase that much, it becomes obvious that using expanders on a limited amount of ports seems to be a good approach.


Devices called "Fan Out" and "Edge" both are expanders. However, only one Fan Out (primary) expander is allowed for a SAS domain (see 4X link to initiator in the middle of the image). There may be up to 128 physical links per edge expander - whether these are merged into wide links and/or linked to other expanders or drives. The result can be pretty complex, but equally powerful and flexible. Source: Adaptec.

Source: Adaptec.
Backplanes are the fundamental building block to any storage appliance that is meant to support hot plugging. So, SAS expanders will always go hand in hand with powerful backplanes (whether these are in a single box or not). Usually, one link is used to attach a simple backplane to the host adapter or to a hop. Expanders with built-in backplanes will of course use multi-link connects.
Three connector types are allowed for SAS: SFF-8484 is a multi-lane internal cable that links a host adapter to a backplane. This can also be achieved by splitting the cables into multiple single SAS connectors on one end (see image below). SFF-8482 is the connector that links a drive to a single SAS port. Finally, SFF-8470 is the external multi-link cabling solution. It has a maximum cable length of six meters.

Source: Adaptec.

This is a SFF-8470 cable for external multi-lane SAS links.

This is a SFF-8484 multi-lane cable. It runs four links via one connector.

Here we have a SFF-8484 cable that is used to attach four SATA drives.
Why do people need all this? Most users certainly will never come close to the SAS topology scheme that we showed above. However, SAS is more than just the next generation interface for professional hard drives, although it is perfectly suitable to set up simple or complex RAID arrays, using one or multiple RAID controller cards. But SAS can do more. It is a serial point-to-point interlink that scales beautifully as you consolidate the number of links between any two SAS devices. SAS hard drives come with two native links by default, so you can link one port from within an expander to a host system, and create a backup data path to another host (or another expander).
Links between SAS adapters and expanders as well as between two expanders can be as wide as the number of available SAS ports. Expanders typically will be rack mount devices that are capable of hosting a large number of drives, and the possible SAS link to the next hop or host adapter is only limited by the capabilities of the expander.
Thanks to the feature rich infrastructure, SAS allows for operating complex storage topologies rather than individual hard drives or storage boxes. In this context, complex does not mean it is difficult to handle. SAS setups can consist of simple disc drive enclosures or expander based structures. Any type of SAS interlink can be added, removed or widened easily depending on your bandwidth requirements, and you may use either sophisticated SAS hard drives, or highest capacity SATA models. In conjunction with powerful RAID controllers, storage arrays can be setup, extended and reconfigured easily - both from a RAID layer and a hardware point of view.
This becomes important if you take into account how professional storage environments increasingly look like. The buzzword is SAN, which stands for storage area network. It describes a decentralized way of organizing enterprise storage with traditional servers, while using physically-outsourced raw storage boxes. Existing Gigabit Ethernet or Fiber Channel network architecture is used to run a slightly modified SCSI protocol, which is encapsulated into Ethernet frames (iSCSI - Internet SCSI). A system running anything between a single hard drive up to complex nested RAID arrays becomes a so-called target that gets mapped into a host system (initiator), thus is treated as if it were a single physical unit.
iSCSI does allow for the creation of a storage strategy, organizing data or managing access to it, but it adds a layer of flexibility by removing directly attached storage from servers and allowing for storage subsystems of any size to become iSCSI targets. Outsourcing storage eliminates storage servers being a single point of failure and it increases hardware manageability. From a software management point of view, storage logically still remains "inside" a server. Initiators and targets may be next to each other, in different rooms, floors or buildings - eventually all that matters is the performance of the IP network connection that you use as your physical SAN carrier. From this standpoint, it is also important to say that a SAN would not accommodate online storage requirements such as databases.
The 2.5" hard drive for professional applications is still a new product category. We took a look at Seagate's first 2.5" Ultra320 Savvio hard drive some time ago, which also left us with a good impression. All 2.5" SCSI drives spin at 10,000 RPM today and thus cannot deliver the same performance level that you would receive from a 3.5" drive at the same spindle speed; the outer areas of a 3.5" model rotate at a faster absolute velocity and thus allow for better data transfer rates.
The advantage of smaller hard drives does not lie in capacity either, as the maximum still is 73 GB today, while 3.5" enterprise class drives hit 300 GB a long ago. In many environments, processing capacity or energy efficiency have become important issues. The more hard drives you deploy, the more performance you can harvest in return - given a suitable infrastructure. While I/O performance is a key issue, a large number of 2.5" drives will be able to outperform most 3.5" drive arrays as well. At the same time, 2.5" drives consume clearly less than half the energy that any 3.5" drive requires. When it comes to performance per watt (I/Os per Watt), the 2.5" form factor delivers very good results.
If capacity is what you need, 3.5" 10,000 RPM drives would not necessarily be our primary choice either, because 3.5" SATA drives provide 66% higher capacity with acceptable performance levels (500 rather than 300 GB per drive). Most hard drive makers offer 24/7 SATA drives today and cost per drive is down to a minimum, so the remaining reliability issues can be attended by buying spare drives for immediate replacement.
The 15,000 RPM drive category still serves a need , because it is the only choice that will offer both the highest transfer rates and I/O performance per drive.

The MAY family represents Fujitsu current 2.5" drive generation for professional use. Fujitsu offers a spindle speed of 10,025 RPM and capacities of 36.7 GB and 73.5 GB. All drives come with 8 MB cache memory and offer 4.0 ms average read and 4.5 ms average write seek time. As already mentioned, the neat thing about 2.5" drives is their reduced power consumption. Usually, one 2.5" consumes at least 60% less energy than a 3.5" drive.


Fujitsu MAX3147RC

MAX is Fujitsu's current high-performance 15,000 RPM hard drive family, so the product family name is appropriate. In contrast to the 2.5" models, the performance drives come with a whopping 16 MB cache and short seek times of 3.3 ms for average read seeks and 3.8 ms for the write seek procedure. Fujitsu has 36.7 GB, 73.4 GB and 146 GB versions of this drive, with one, two and four platters.
Fluid dynamic bearings have been deployed to enterprise hard drives as well, so this model runs noticeably quieter than earlier-generation 15k hard drives. Still these drives should never be operated without proper ventilation, which hard drive enclosures usually offer.



Hitachi Global Storage Technologies hasits own enterprise drive for high-performance environments. The UltraStar 15K147 runs at 15,000 RPM and comes with 16 MB cache like the Fujitsu drives , but it has different platter configurations. The 36.7 GB model uses two rather than one platter, and the 73.4 GB drive has three rather than two. Although this indicates a lower data density, the design, in fact, avoids the use of the inner, slower area of the magnetic platters. As a result, the distance that the heads have to cover may be smaller, which can result in better average access time.
Hitachi also has 36.7 GB; 73.4 GB and 147 GB models with a purported read seek time of 3.7 ms for all models.



Although Maxtor nowadays is part of Seagate, it will keep its own product lines. The vendor offers 36, 73 and 147 GB models, all of which run at 15,000 RPM and have 16 MB SDRAM cache memory. The company claims the devices have an average seek time of 3.4 ms reading and 3.8 ms writing.



Cheetah has been a synonym for high-performance hard drives for a long time. The enterprise product nomenclature actually worked well enough for Seagate to port its Barracuda name into the desktop space, which resulted in the first 7,200 RPM desktop hard drive in 2000.
There are 36.7 GB, 73.4 GB and 146.8 GB model versions available. All of these run at 15,000 RPM and have 8 MB cache. They feature 3.5 ms read and 4.0 ms write seek time.



Unlike SATA controllers, SAS components will only be found on server class motherboards or as separate add-in cards for PCI-X or PCI Express. If you take another step forward and look at RAID controllers (Redundant Array of Inexpensive Drives), these are mostly sold as add-in cards due to their complexity. RAID cards do not only include the actual controller, but they have a hardware accelerator for parity calculation (called the XOR engine) as well as some cache memory. A limited amount is either soldered onto the controller card (usually up to 128 MB), or some cards allow for installing a DIMM or SO-DIMM instead.
You should identify your requirements first when selecting a host adapter or RAID controller, as there is an increasing range of products. Simple multi-port host adapters will be relatively cheap, while powerful RAID cards usually are a considerable investment. Think of where to place your storage devices as well: Using storage boxes requires at least one external connector. Rack mount server scenarios sometimes require low profile add-in cards.
If you think RAID, determine whether or not you need hardware acceleration. Some RAID cards burden the system processor(s) with XOR calculation for RAID 5 or 6; others have an XOR offload engine. Accelerated RAID is required for environments in which a server is not dedicated to storage only, such as databases and Web services.
All the host adapter cards that we present on the following pages support 300 MB/s speed per SAS port and are as flexible as described above when it comes to storage infrastructures. External ports are pretty much standard already, and both SAS and SATA hard drives are supported. All three cards use the PCI-X interface, but PCI Express versions are already in development.
Although all cards in this article are eight port models, this does not limit the maximum number of disk drives. Using a SAS expander (externally), you can attach all types of storage appliances to any of these adapters. As long as the quad link connection is fast enough for your applications, up to 122 drives are supported by the SAS protocol. Due to the performance impact of parity added RAID 5 or RAID 6, typical RAID setups will not be able to saturate the bandwidth of these quad links even when multiple drives are being used.

The 48300 is a SAS host bus adapter that was designed for the PCI-X bus. Still, PCI-X is the dominating interface in the server space, although more and more motherboard products come with PCI Express core logic.
PCI-X at 133 MHz as used by the Adaptec SAS 48300 provides an interface throughput of 1.06 GB/s, which is definitely fast enough, unless the PCI-X bus is being shared by multiple devices. If a lower speed device is plugged in, all other PCI-X devices have to switch down the clock speed to match this lowest common bus speed. Making sure that a PCI-X controller has a PCI-X bridge of its own is the safe way to maintain maximum bandwidth.
Adaptec offers the SAS 4800 for the low- to mid-range server and workstation space. At a MSRP of $360, it is reasonably priced and it supports Adaptec's HostRAID feature, which is a system powered basic RAID implementation. RAID levels 0, 1 and 10 are offered. The card features one internal SFF8470 quad link connector and a second SFF8484 connector featuring four external SAS links, for a total of eight links.
The card fits into a 2U rack mount server with a low-profile bracket included. It also comes with a driver CD, a quick installation manual as well as a four-port SAS fanout cable that connects up to four system drives to the adapter board.


Adaptec SAS RAID 4800

SAS RAID4800SAS is Adaptec's answer to more sophisticated SAS storage requirements and thus can be used to power application servers, streaming servers, security applications and so on. Again this is an eight-port controller card, with one external quad link SAS connector and two internal quad link SAS ports. However, if the external port is used, only one of the internal link connectors may be used.
This card also is a PCI-X 133 model, which offers sufficient bandwidth to power even the most demanding RAID setups.
In regard to RAID, the SAS RAID 4800 easily out performs its little brother; RAID levels 0, 1, 10, 5, 50 are supported by default, given you're running an adequate amount of drives. Unlike the 48300, Adaptec bundles two SAS fanout cables, so you can attach eight drives inside your server system right from the start. In contrast to the 48300, this card requires a full height PCI-X slot
If you decide to upgrade to Adaptec's Advanced Data Protection Suite, you will receive the double redundancy RAID modes 6, 60 as well as enterprise class features such as the option for a striped mirror drive (RAID 1E), hot spacing (RAID 5EE) and copyback hot spares. The Adaptec Storage Manager is the browser enabling interface that is used to administer all Adaptec storage adapters.
Adaptec offers drivers for Windows Server 2003 (and x64), Windows 2000 Server, Windows XP (x64), Novell Netware, Red Hat Enterprise Linux 3 and 4, SuSe Linux Enterprise Server 8 and 9 and FreeBSD.


SAS player LSI Logic sent us its SAS3442X PCI-X host adapter, which is the direct competitor to Adaptec's SAS 48300. It comes with eight SAS ports that are distributed to two quad link ports, using LSI's SAS1068 controller. One of these is available for internal use, the other one allows for running external DAS (Direct Attached Storage) devices. The board connects to your server system via PCI-X 133.
As usual, 300 MB/s transfers are supported both for SATA and SAS drives. There are 16 LEDs on the controller board. Eight of them are basic activity LEDs, while the other eight are fault LEDs that are used to display problems.
LSI's SAS3442X is a low profile PCI-X card and, thus, will fit into any 2U rack mount server.
The driver support includes native Linux drivers, Netware 5.1 and 6, Windows 2000 and Server 2003 (x64), Windows XP (x64) and Solaris up to 2.10. Different than Adaptec, LSI does not support any kind of RAID mode with this card.



Adaptec Storage Enclosure 335SAS

The 335SAS is a four-drive enclosure for SAS or SATA drives, but it must be connected to a SAS controller. Thanks to an attached 120 mm fan, the drives will always be sufficiently ventilated. Two Molex connectors are required to provide power to the enclosure.
Adaptec includes an I2C cable that can be used to manage the enclosure via a suitable SATA controller. However, this is not possible with SAS drives. An additional LED cable is used to support drive activity signaling, but again, for SATA only. An internal quad link SAS cable is part of the package, so linking the enclosure to your controller will only require this cable. Should you want to attach single SATA, drives you can use the SAS to SATA fanout cable.
At a SRP of $369, the 335SAS is not a bargain. But it does offer a decent environment for hard drives that you want to remain reliable.



Adaptec SANbloc S50 JBOD

SANbloc S50 is an enterprise level 12-drive 2U rack mount solution for SAS controllers. This is probably the best example of enterprise scale SAS applications. The 12 drives may be either SAS or SATA drives, or a mixture of both. The included expander logic uses one or two SAS quad links to hook the S50 up to your host adapter or RAID controller. It is obvious that we are talking about a professional solution as it is based on two redundant power supply units with independent power inlets.
If you're already using an Adaptec SAS host adapter, you will be happy to hear that you can actually use the Adaptec Storage Manager to control the S50 and its drives. Using SATA hard drives at 500 GB each, the maximum of 12 drives will result in 6 TB of storage space, while 300 GB SAS drives will result in 3.6 TB. At two quad SAS links to the host adapter, the bandwidth is 2.4 GB/s, which should definitely be more than enough for any type of array. Putting all 12 drives into a RAID 0 array would result in maximum transfer rates of around 1.1 GB/s. A slightly different version with two independent SAS I/O modules will be available in the middle of this year.
The unit includes automatic temperature monitoring and includes automatic fan speed control. Yet, it was noisy enough for us to ban it from our offices after our tests were completed. Drive failure is communicated to the controller via SES-2 (SCSI Enclosure Services) or over a physical I2C interface.
Different from drive operating environment temperatures (41-131°F or 5-55°C), the enclosure may operate in between 32-104°F (0 and 40°C).
At the beginning of our tests, we reached burst transfer speeds of only 610 MB/s only. After changing the interface cable between the S50 and the Adaptec host controller, we finally managed to hit 760 MB/s. We found seven hard drives to saturate the system in RAID 0. Adding more drives would not allow for higher throughput.





| System Hardware | |
|---|---|
| Processor(s) | 2x Intel Xeon Processor (Nocona core)
3.6 GHz, FSB800, 1 MB L2 Cache |
| Platform | Asus NCL-DS (Socket 604)
Intel E7520 Chipset, BIOS 1005 |
| RAM | Corsair CM72DD512AR-400 (DDR2-400 ECC, reg.)
2x 512 MB, CL3-3-3-10 Timings |
| System Hard Drive | Western Digital Caviar WD1200JB
120 GB, 7,200 rpm, 8 MB Cache, UltraATA/100 |
| Mass Storage Controller(s) | Intel 82801EB UltraATA/100 Controller (ICH5)
Promise SATA 300TX4
Adaptec AIC-7902B Ultra320
Adaptec 48300 8 port PCI-X SAS Controller
Adaptec 4800 8 port PCI-X SAS Controller
LSI Logic SAS3442X 8 port PCI-X SAS Controller
|
| Storage Systems | Adaptec Storage Enclosure 335SAS
Internal 4-bay hot-swap drive chassis Adaptec SANbloc S50 JBOD
|
| Networking | Broadcom BCM5721 On-Board Gigabit Ethernet NIC |
| Graphics Card | On-Board Graphics
ATI RageXL, 8 MB |
| Benchmarks | |
| Performance-Messungen | c’t h2benchw 3.6 |
| I/O Performance | IOMeter 2003.05.10
Fileserver-Benchmark Webserver-Benchmark Database-Benchmark Workstation-Benchmark |
| System Software & Drivers | |
| OS | Microsoft Windows Server 2003 Enterprise Edition, Service Pack 1 |
| Platform Driver | Intel Chipset Installation Utility 7.0.0.1025 |
| Graphics Driver | Default Windows Graphics Driver |
3.5" Ultra320 SCSI Data Transfer Diagrams






2.5" SAS Data Transfer Diagrams




Interface Performance










After lining up many new SAS hard drives, three representative controller solutions and two storage appliances, it is obvious that it was entirely worth waiting for SAS. If you read the technical documentation about SAS, you'll understand why. Not only has the serialized version of SCSI been transformed into a fast, convenient and easy-to-use storage interface, but it has also reached a level of scalability and sophistication that makes Ultra320 SCSI look like a stone age leftover.
Besides, interoperability is very smooth. If you were toying with the idea of buying professional SATA equipment for your server(s), you really should really think about SAS instead. Any SAS controller and enclosure available is compatible with either SAS or SATA hard drives and is thus suitable for either high-capacity SATA or high-performance SAS environments - or both at the same time.
Externally-operated storage devices are the next advantage for SAS: While SATA hardware either used proprietary cabling or is based on a single SATA/eSATA link, SAS links scale up in units of four SAS connections. This allows for matching bandwidth to the actual requirements rather then being stuck at UltraSCSI's 320 MB/s or SATA's 300 MB/s. Moreover, SAS expanders can be used to cascade SAS devices, giving administrators a powerful operating range.
Any future performance benefits will also only be available for SAS devices. We expect the industry to reduce its UltraSCSI offers to a level at which it does no more than support existing UltraSCSI deployments. All new hard drives and the latest enclosures and appliances, as well as interface speed upgrades to 600 and eventually 1,200 MB/s, are geared for SAS.
So, what storage infrastructure should you buy today? With the availability of SAS, the days of UltraSCSI are numbered. The serialized version does everything better, so whether to get UltraSCSI or SAS becomes obvious. The decision becomes more difficult if you choosing between SAS or SATA.However, as long as your storage requirements are clearly assessable on the long term, SAS components likely offer the better bang for the buck, which is certainly the case for high capacity near-line storage scenarios. Indeed, for maximum performance or big time scalability, there is no alternative to SAS.